1. Field of the Invention
The present invention relates to a hot wire type flow sensor for detecting a flow rate of fluid by using a resister as a hot-wire.
2. Discussion of Background
There has been known a flow sensor of such a type that a flow rate of fluid is detected by utilizing a state of thermal equilibrium by means of a bridge circuit including a heating resister which is disposed in a fluid passage.
Description will be made as to a conventional air flow sensor having a platinum wire as the heating resister.
FIG. 5a is a cross-sectional view of a detecting means in the conventional hot wire type flow sensor, and FIG. 5b is a side view of the sensor. In FIGS. 5a and 5b, a plurality of hot wire supporters 2 are attached on the inner surface of a cylindrical housing 1, and a hot wire R.sub.H is extended by the hot wire supporters 2 in a plane perpendicular to the flowing direction of air. A symbol R.sub.C designates an air temperature sensor provided in the housing 1.
FIG. 6 is a diagram showing a bridge circuit including the hot wire R.sub.H and the air temperature sensor R.sub.C and a circuit for controlling temperature so that the bridge circuit maintains a state of thermal equilibrium. The bridge circuit is constituted by resisters R.sub.1, R.sub.2, the hot wire R.sub.H and the air temperature sensor R.sub.C, and both input terminals of a differential amplifier 101 are connected respectively to connecting points b, f in the bridge circuit.
The output terminal of the differential amplifier 101 is connected to the base of a transistor 102; the emitter of the transistor 102 is connected to a terminal a of the bridge circuit; and the collector of the transistor 102 is connected to the positive terminal of a d.c. power source 103.
The operation of the flow sensor having the above-mentioned construction will be briefly described. When voltages appearing at the connecting points b, f in the bridge circuit become equal, the bridge circuit reaches an equilibrium state. At this moment, an electric current I.sub.H corresponding to a flow rate of air in the air passage flows in the hot wire R.sub.H, whereby a voltage V.sub.O at the connecting point b is represented by I.sub.H.R.sub.2, and the voltage V.sub.O is used as a signal of the flow rate.
In the conventional hot wire type flow sensor, a wire having a diameter as thin as about 70 .mu.m is used for the hot wire R.sub.H in order to increase sensitivity to the change of the flow rate. On the other hand, the supporters 2 should withstand a force caused by an air flow in order to maintain the hot wire R.sub.H at a predetermined position. For this, the supporters 2 are made of a material having a heat capacity greater than the hot wire R.sub.H. Accordingly, the supporters 2 having a large heat capacity constitute a part of the heat sensitive element since the air flows uniformly inside the cylindrical housing 1. This results in reducing sensitivity for detecting the flow rate of air.